The present invention relates to an aqueous acidizer composition and the use of the composition in the removal of scale from fresh water production equipment. More particularly, the present invention relates to an acidizer composition containing a blend of three acids of varying strengths, with each component of the composition being carefully selected to provide a timed release effect so as to prolong the activity of the composition in scale removal over an extended period of time while simultaneously minimizing pitting and fatigue of the metallic equipment or its components.
Various compositions for use in water treatment or the removal of scale from water storage and transfer equipment as well as cooling towers, boilers and some types of fresh water production equipment, include the compositions as described in the following U.S. Pat. No. 3,235,324 to Merriman; U.S. Pat. No. 3,335,085 to Hronas; U.S. Pat. No. 3,424,688 to Boiko et al; U.S. Pat. No. 3,458,354 to Reich; U.S. Pat. No. 3,969,255 to Connelly; U.S. Pat. No. 4,199,469 to Walzer; and U.S. Pat. No. 4,222,886 to Connelly. Various of these patents disclose the use of hydrochloric acid along with additional components and, in particular, U.S. Pat. No. 4,199,469 discloses the use of hydrochloric acid along with isopropyl alcohol and additional acid components. The use of isopropyl alcohol has been restricted to its ability to kill microorganisms, remove bio-organic residues such as algae, and as a solvent for various organic components of formulations such as described in U.S. Pat. No. 3,335,085.
In the removal of scale from certain types of fresh water production equipment, sometimes referred to as "watermakers", strong mineral acids such as sulfuric, nitric and perchloric acids have been found to cause serious damage to the equipment. This is due to the fact that both the hydrogen ions and the oxidizing anions of these acids tend to cause severe metal pitting at the acid concentrations which are required for scale removal. In such treatment for scale removal, acid concentrations of at least 0.6 M are generally necessary. Although hydrochloric acid does not contain an oxidizing anion, the chloride ion in high concentration readily coordinates copper, nickel and other transition metal ions, thus increasing the probability of metal pitting. This effect coupled with the corrosive properties of hydrogen ions in high concentration prohibits the use of hydrochloric acid alone to descale watermakers. To be effective, a solution concentration in excess of 0.6 M is desirable.
The use of phosphoric acid to descale watermakers is not feasible. Not only is it a relatively weak acid, thus prolonging the time required for acidization, but also serious difficulties arise when using phosphoric acid in watermakers. Watermakers are primarily restricted to marine or related use in which water processed contains an extremely high level of calcium, magnesium, iron and other heavy metal ions which readily react with phosphoric acid to produce phosphate scales in these units. As there is always a residual amount of sea water in a watermaker during acidization, phosphoric acid creates a scale problem rather than alleviating one.
In the group of moderately strong inorganic acids which are conventionally employed in scale removal, sulfamic acid has previously been the most popular of such acids employed in the descaling of water production equipment. It has been found that sulfamic acid does indeed remove phosphate and carbonate scale without causing serious damage to the metal unit. The K.sub.a for sulfamic acid is equal to 1.03.times.10.sup.-1. The use of sulfamic acid, however, has presented the following major disadvantages in the watermaker descaler process: (1) Sulfamic acid must be sold dry requiring "on site" mixing. This is very time consuming due to the relatively low solubility of the acid in water under ambient conditions. Aqueous solutions of the acid are unstable upon standing. Over a short period of time sulfamic acid in water hydrolyzes with the formation of NH.sub.4 HSO.sub.4. This prohibits the marketing of the acidizer as an aqueous solution; (2) Because of the low solubility of sulfamic acid in water, successive acidizations and flushings of the watermaker are frequently necessary for complete scale removal. This again is quite time consuming; (3) With even moderate to low temperature fluctuations within the watermaker, sulfamic acid frequently precipitates out of solution during acidization, clogging the watermaker tube bundles and other very "difficult to clean" components; (4) Calcium, magnesium and other metal sulfamate salts that are produced during the acidization process are quite water insoluble and thus precipitate within the watermaker tube bundles and other "difficult to clean" components. This poses a unit "clogging" problem similarly to point (3) above; (5) Sulfamic acid will cause some metal pitting of 90% copper-10% nickel alloy, and thus evaporator parts which are repeatedly exposed to the acid during descaling can experience considerable damage.
By the present invention, there is provided an improved composition for use in the removal of scale from fresh water production equipment. The composition of the present invention includes a blend of three acids of varying strengths carefully selected to provide an acidizer having a timed release effect. This timed release effect significantly enhances maximum scale removal with minimal or insignificant pitting of the metal evaporator or other equipment. The composition also contains a surfactant, preferably isopropyl alcohol, which taken together with its reaction products with the acid components, serves to accelerate the descaling process. An acid-base indicator and water also form a part of the present composition.